1. Field of the Invention
The present invention relates to an image forming apparatus, capable of performing both-sides printing on a recording sheet, a method therefor, and a program.
2. Description of the Related Art
In a case where a conventional image forming apparatus performs both-sides printing on recording sheets, a both-sides printing method is known that as an initial step, an image is formed on the first surface of each of some recording sheets and thereafter as a subsequent step, an image is alternately formed on the first surface of a recording sheet and second surface of a recording sheet (for example, see, U.S. Pat. No. 4,935,786).
There exists an image forming apparatus capable of connecting to various kinds of post-processing apparatuses. In a case where the image forming apparatus is connected to a post-processing apparatus performing a stapling processing and/or a post-processing apparatus performing a sorting processing, a processing capacity for each of the post-processing apparatuses per unit time is made higher than an image forming capability of the image forming apparatus so as to prevent the image forming apparatus from being kept waiting for an image forming processing thereof. On the other hand, in a case where the image forming apparatus is connected to a post-processing apparatus performing a time-consuming processing on the assumption of being performed a both-sides printing mode such as a bookbinding function, the post-processing apparatus is made to have a capability half of or more than half of an image forming capability of the image forming apparatus in a one-side printing mode so as to substantially prevent the image forming apparatus from being kept waiting for the image forming processing thereof.
In the meantime, recently, the image forming apparatus is required to improve image quality for the both-sides printing thereof, and a problem is pointed out that images formed on the first and second surfaces have different sizes from each other because a recording sheet shrinks during thermal fixing performed along with the image formation on the first surface of the recording sheet. In order to cope with this problem, a method is proposed to switch a rotational speed of a polygon mirror for the image formation between the first and second surfaces (for example, see, U.S. Pat. No. 6,839,078).
A high-speed image forming apparatus requiring high-quality images needs to have a configuration to change the rotational speed of the polygon mirror during the both-sides printing. However, it needs a lot of time to change the rotational speed of the polygon mirror because the polygon mirror is made to have a large inertia to stably rotate at a high speed. As a result, in a case where the image formation on the first and second surfaces of a recording sheet are alternately performed sheet by sheet, it is necessary to perform a speed-changing processing of the polygon mirror at every such occasion, thereby making the image forming processing itself of the image forming apparatus becomes slower.
This problem can be solved by performing the image formation on the first surfaces of a plurality of sheets at one time and subsequently performing the image formation on the second surfaces at one time instead of alternately performing the image formation on the first and second surfaces. This is because, if such configuration is employed, the rotational speed of the polygon mirror changes for less number of times, the image forming apparatus can reduce a time period for the image forming processing.
However, the image forming apparatus having the configuration as described above successively performs the image formation on the second surfaces of the plurality of recording sheets. Accordingly, in a case where a post-processing is performed by a post-processing apparatus having a processing capability half of a processing capability of the image forming apparatus in a one-side printing mode, the post-processing apparatus may cause the image forming apparatus to be kept waiting for the image forming processing thereof. In addition, it becomes necessary for the post-processing apparatus to be provided with a buffer for storing the recording sheets so that the post-processing can be done while the image forming apparatus is performing the image formation on the first surfaces.
On the other hand, when the image formation is performed on thick sheets, the number of the sheets for image formation per unit time may sometimes be reduced so that a fixing unit can apply sufficient heat to the thick sheet. In such case, it is less likely to cause the image forming apparatus to be kept waiting for the image forming processing even where the image formation is performed alternately on the first and second surfaces to repeatedly change the rotational speed of the polygon mirror, and even where a time-consuming post-processing is executed, waiting time for the processing can be reduced.